June 23--H&R Block CEO Bill Cobb sold 750,000 Block shares this week in a move to diversify his assets ahead of retirement from the company at the end of next month.

The value of the sales topped $13.2 million.

Cobb's sales came about a month ahead of his retirement after six years at the helm of the Kansas City-based tax preparation company. General counsel Tom Gerke will serve as interim CEO while the company searches for a permanent top officer.

CEOs and other insiders are limited as to when they can buy or sell shares in the market, given that they often have material information that is not known publicly. Cobb's sale came after the company reported higher-than-expected profits June 13.

For many top corporate executives, stock holdings in the company where they work can account for a large share of their net worth. They typically limit sales while in office as a show of faith in the company's future and their ability as CEO to make the shares more valuable.

A filing with the Securities and Exchange Commission on Thursday showed Cobb's transactions took place Tuesday and Wednesday.

One sale involved stock options Cobb had received as part of his compensation in previous years. When granted, Cobb's options allowed him to buy 500,000 shares at $19.14 a share, which he did this week. He then sold them for $30.18 in the market on the same day, making his proceeds $5.52 million.

The options had vested to his control in 2013 and 2014 and were set to expire in 2021.

Cobb also sold 250,000 other shares, at about $30.84, generating proceeds of more than $7.7 million. It was unclear from the filing how much those shares had cost Cobb.

Tom Collins, head of Block's corporate communications, said the sales represented less than 25 percent of Cobb's holdings in Block.